Burnside High School second principal Sandra Sidaway said police told them the sales were not on school grounds and were made well outside school hours. For this reason, it was unable to take action against the student.

"Any time there's drugs on-site, we take action. The parent was very clear [her child] had not been offered it in the school," she said.

Sidaway had not been aware it was K2 being sold.

Principal Warwick Maguire said the school took a "hard line" on any sale of prohibited substances.

"If they were selling synthetic cannabis at school and we found out about it, the student would obviously be dealt with appropriately," he said.

Seal, who is running an operation asking Christchurch dairies to stop selling synthetic cannabis, said children getting access to the legal highs was "a huge concern".

Parents needed to "wake up" to the synthetic cannabis issue.

"A lot of parents are just naive. They need to watch their kids, what they are doing and their behaviour. It could be their kids are smoking this."

298 Youth Health Centre founder Sue Bagshaw said they knew of children as young as 13 taking K2, which they had got from friends.

"It's worse than cannabis. You've got no idea what's in it."

Christchurch Hospital emergency department specialist Dr Paul Gee said patients admitted to hospital after taking synthetic cannabis-type drugs were mostly young adults but also included school pupils.

He said it was a "concerning trend".

"Many young people do not appreciate the risk of taking these drugs. Each use may cause harm - each is a roll of the dice."